1. Field of the Invention
The invention generally relates to a circumferential seal between a seal ring and a runner capable of large axial translations with respect to the seal. Specifically, the invention includes a plurality of hydrodynamic grooves disposed along the outer circumference of a runner either parallel or diagonal to the direction of rotation. Hydrodynamic grooves are further arranged into separate and distinct groove sets about the runner. Each groove set includes at least two grooves which either separately or jointly exert a hydrodynamic lifting force onto the inner diameter of the seal ring regardless of their position relative to the runner.
2. Background
There are many applications wherein housings are provided with a plurality of interior sections having rotating parts passing there through, wherein one of the interior housing sections must be isolated from another by means of a seal system. In gas turbine applications, for example, it is critical that the lubricant contained within a lubricant chamber of the housing be sealed from an adjacent fluid or gas side of the seal. This is especially true along a rotatable shaft which often passes from the lubricant side of the seal to the fluid side. In an aircraft engine, these sump seals are used to separate ambient areas of high pressure air, e.g. the gas side, from an oil wetted area at lower ambient pressures, e.g. the lubricant side. These seals prevent oil leakage from the lower pressure compartment and minimize the flow rate of hot air from the high pressure area to the oil wetted compartment.
Leakage of liquids from the lubricant side into the gas side adversely affects performance of the equipment where a seal is used. In the case of an aircraft engine, oil leakage across the seal into a hot air side may cause oil coking or an engine fire. More specifically, when an oil lubricant is used, mixing the oil with the gas could result in formation of oil coke, a byproduct of oil heated to an elevated temperature, which chemically alters the oil and is detrimental to the gas turbine. Oil coke can foul seal surfaces reducing the integrity of the seal and preventing proper bearing lubrication within the lubricant sump. Accordingly, it is important in similar applications, not just aircraft engines, that the lubricant be isolated within a lubricant sump and that the seal around the rotating shaft not allow the lubricant to escape the sump. Seals in such applications may comprise either circumferential seals or face-type seals; however, circumferential shaft seals are the most widely used under the above conditions.
The term circumferential seal broadly describes a generic type of sealing device used widely, inter alia, on aircraft engine applications. FIGS. 1a and 1b show a standard seal assembly 1 disposed about a runner 2 between a low pressure region 3 and a high pressure region 4. The seal assembly 1 supports a seal ring 6 about the runner 2 and typically includes a seal housing 5, a retaining ring 7, a back plate 8, a plurality of compression springs 9 disposed about the seal ring 6, a garter spring 10, a cavity 43, and an anti-rotation pin 14. A lift pad 11 along the seal ring 6 forms a circumferential seal with the runner 2 at the sealing radius 15 and could include a dead-end bore groove 26 and a bore dam 13 to improve sealing performance. A second surface 12 along the seal ring 6 provides a face seal with the seal housing 5.
FIGS. 2a and 2b show face and bore views, respectively, of a standard ring segment 16. A plurality of ring segments 16 typically comprises a seal ring 6. Each ring segment 16 is composed primarily of carbon and/or graphite and is arranged circumferentially around a runner 2 to form a continuous, relatively stationary seal ring 6. Each ring segment 16 includes a tongue 17 and a socket joint 18 which overlap between two adjacent ring segments 16 to restrict leakage. The related arts describe sealing rings with one or more pockets or similar structures along the bore thereof. The related arts do not provide such pockets along the outer circumferential surface 19 along the runner 2 shown in FIG. 1a. 
Most current circumferential seals utilize a variant of the circumferential seal illustrated in FIGS. 1a, 1b, 2a, and 2b to address the sealing requirements between a low pressure liquid compartment and a high pressure gas compartment. In one example, Pope discloses in U.S. Pat. No. 5,145,189 a sealing ring with a shallow groove which redirects pressurized air to a plurality of deeper vent grooves. In another example, Hwang discloses in U.S. Pat. No. 6,145,843 a sealing ring with shallow lift pockets in fluid communication with a high pressure region by a plenum chamber.
The position of grooves along the bore of a sealing ring is problematic, particularly in higher-performance turbine engines. First, sealing rings are typically composed of carbon graphite and as such are prone to surface wear which compromises shallow hydrodynamic grooves along a ring. Second, the design of and operating conditions within such engines often cause the runner to widely translate along the axis of the engine. For example, axial translations in the range of a quarter of an inch are possible by a runner in some applications. Large relative movement between a runner and a sealing ring with conventional groove arrangements aligned at a single axial location allow fluid within the grooves to vent in an uncontrolled fashion. The result is a reduction or loss of the hydrodynamic lifting force exerted by the grooves onto the runner. A less robust lifting force is more likely to allow contact between the runner and sealing ring. Any such contact wears the bore surface along the sealing ring, reducing the depth and performance of the grooves over time.
As is readily apparent from the discussions above, the related arts do not include a circumferential seal which avoids the performance problems associated with seal systems that include a runner capable of large axial translations and a sealing ring with grooves along its bore.
Accordingly, what is required is a circumferential seal which maintains the lift properties between a seal ring and a seal runner during large axial excursions of the seal runner.